1. Field of the Invention
This invention relates to a method for producing receptacles from a thermoplastic resin foam sheet. More specifically, this invention relates to a very easy and efficient method for producing receptacles having a deep bottom, receptacles whose mouth and bottom are different in diameter, and receptacles having complicated shapes, using a uni-directionally shrinkable thermoplastic resin foam sheet which, upon application of heat, shrinks in one direction to a greater extent than in directions perpendicular to that direction.
2. Description of the Prior Art
Previously, the manufacture of receptacles from thermoplastic resin sheets has been effected exclusively by vacuum molding or compression-molding of a flat foamed sheet using a pair of molds in which the mold cavity corresponds to the shape of the desired receptacle. Since the receptable obtained by such a method is a product in which the side wall and the bottom are integrally molded from the thermoplastic resin foam sheet having good heat insulation, one can hold it with the hand even when a very hot liquid such as water or beverage is poured into it. However, since the molding involves heat-softening the thermoplastic resin foam sheet and forceably elongating the foam sheet, and since the elongation of the foam sheet is limited, the peripheral side wall or the bottom of the receptacle becomes thin and its strength decreases. Consequently, receptacles having a height larger than the diameter of its open end, i.e., receptacles with deep bottoms cannot be obtained. Furthermore, with such a conventional technique, it is almost impossible to obtain receptacles of complicated shapes.
In Japanese Laid-Open Patent Publication No. 31270/73 a method is proposed which is an improvement over the above described conventional method and which makes it possible to produce receptacles having a deep bottom without a bonding adhesive. Briefly, this method comprises bending into a cylindrical form a frustosectoral or rectangular thermoplastic resin foam sheet which when bent in a cylindrical form, has a substantially larger height than its diameter, sealing the side edges of the sheet to form a lapped seam and a longitudinal side wall, and bonding a bottom plate consisting of the same kind of thermoplastic resin foam sheet as the side wall to the inner bottom part of the side wall thereby to form a cylindrical receptacle. Since according to this method, a frustosectoral or rectangular thermoplastic resin foam sheet is bent into a cylindrical form and the edges of the foam sheet are merely heat-sealed, it is possible to produce a desired cylindrical receptacle with a deep bottom and having a larger height than its diameter. However, when, for example, a cup having a mouth with a different diameter from that of its bottom is to be formed by such a method, the foam sheet which constitutes the side wall of the receptacle assumes a complicated shape such as a frustosectoral shape. Consequently, it is time-consuming to punch out a foam sheet for producing the side wall of such a receptacle from a large-sized stock of foam sheet, and the efficiency of using the stock is low and there is a great waste of the foam sheet material. Thus, this method is very disadvantageous from an economic point of view. Such a defect becomes even greater as the shape of the cup becomes more special. For example, in the production of a receptacle having a polygonal mouth edge and a circular bottom or a receptacle having a diameter abruptly decreasing from the central part of the side wall toward the bottom, not only is the efficiency of using the stock of the foam sheet reduced, but also it is extremely difficult to form receptacles of the desired shape.